In recent years, lactides which are dimeric cyclic esters obtained by thermally decomposing an oligomer of a hydroxycarboxylic acid, e.g., lactic acid, have been attracting attention as a material for biodegradable polymers.
A number of reports have hitherto been made with reference to production of lactides by thermal decomposition of hydroxycarboxylic acid oligomers. For example, German Patent 1083275 discloses a process using a metal of the group IV, V or VIII of the periodic table or a compound thereof (group 4, 5, 8, 9, 10, 14 or 15 according to the latest notation) as a catalyst; German Patent 3708915 describes a process using tin or a tin compound as a catalyst; German Patent 1234703 suggests use of a titanium tetraalkoxide catalyst; and German Patent 250413 teaches use of a zinc oxide catalyst.
Every known process makes it a chief aim to obtain a lactide having a high optical purity. However, polymers prepared from an optically active lactide having a high optical purity exhibit limited characteristics because of the high crystallinity and lack in softness demanded for use as general-purpose polymers as recently expected.
In order to control crystallinity of the lactide polymers, such a troublesome means as mixing a highly optically active lactide with another separately synthesized optically active lactide or an optically inactive lactide has been taken. In other words, the known processes for producing lactides of high optical purity are not always deemed advantageous for production of biodegradable polymers with broader latitude in physical properties to which the latest attention is being directed. It has thus been demanded to develop a process for producing lactides having arbitrary optical purity.
On the other hand, since a crude lactide resulting from thermal decomposition of a hydroxycarboxylic acid oligomer contains impurities, such as lactic acid, and is unsuitable as a starting material for polymers, it should be purified by recrystallization and the like. For example, recrystallization from a diethyl ether/petroleum ether mixed solvent or a substituted benzene solvent (e.g., toluene) is reported in Angew. Chem., Int. Ed. Engl., Vol. 18, p. 310 (1979) or British Patent 1122229, respectively.
Further, Japanese Patent Publication (examined) No. 6673/76 discloses a purifying process comprising adding a molten crude lactide to t-amyl alcohol or t-butyl alcohol at 30.degree. to 50.degree. C. to prepare a slurry having excellent storage stability and further adding the same alcohol to the slurry to conduct recrystallization.
However, any of the conventional purifying processes attains a recovery as low as about 70% and are not regarded industrially advantageous.